In preparing single still images or a frame of a video sequence for storage or for transmission, the objects contained within the image or frame are divided into foreground and background planes. These planes are referred to as alpha planes.
The traditional method of encoding the multiple alpha planes of the still image or frame of video sequence composed of multiple objects is to compress each plane independently. One common method of encoding the alpha masks of various objects in a frame is to encode the contour of each object's alpha mask independently. If the alpha mask of an object is in gray scale, the intensity of the interior of alpha mask is also encoded for that object.
This approach is inefficient, however, since it does not take advantage of the correlation between the alpha planes. Further, in some computer graphic application, the background layer often does not have an alpha mask associated with it as it was held to exist everywhere on the image and was included as appropriate by the foreground objects or planes.
The necessity for the background layer to have an alpha mask associated with it arises in low bit rate coding applications where the areas of the background not visible in the scene will not be transmitted. Some current video models, for example, require that explicit alpha masks for the background be available so that the encoder can correctly fill in the regions in which the background exists before transformation with a discrete cosine transform. Thus, the decoder also requires that the background alpha mask be transmitted to it so that the background layer can be decoded correctly. By way of example, with the current MPEG 4 low bit rate video verification models, the losslessly encoded alpha mask of the background layer of a test sequence requires approximately 2500 bits per frame. For a sequence compressed at 128 kilobits per second, with 10 frames per second, the alpha mask of the background layer alone constitutes about 20% of the final bit rate. Additionally, the coding of each alpha mask independently is inefficient since it does not take advantage of the correlation between the alpha planes intersection or overlap, in addition to the masking effect of the foreground planes.